At the time the belief was that COVID spread largely through touching infected surfaces. That combined with the fact that there was a massive shortage of masks

At the time the belief was that COVID spread largely through touching infected surfaces. That combined with the fact that there was a massive shortage of masks meant that they warned against using masks. Also, the role of asymptomatic carriers wasn’t all that clear. And it was probably the right call based on the information we had then (the real question is what the US government was doing for the many months notice it had and why it did not use its powers to deploy companies to manufacture masks or as in the case of GM agree to pay the at cost rates they were offering the US government for manufacturing PPE and ventilators). Once it became clear that asymptomatic carriers and spreaders existed, and that the majority of spread was through water droplets emitted during actions like speaking and singing (unlike most other viruses where it’s usually sneezing and coughing), the advice was changed rapidly since regular cloth masks could be effective for this purpose. This wasn’t a failure of expertise but was the natural consequence of facing a novel threat that was moving faster than we were learning about it.